An organic light emitting diode (OLED) typically include a cathode and an anode, and light emitting units between the cathode and the anode. An OLED can be a single unit OLED or a tandem OLED. A single unit OLED has only one light emitting unit, which includes a hole injection layer (HIL), a hole transport layer (HTL), an emitting material layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL). A tandem OLED has multiple light emitting units connected in series between the cathode and the anode.